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In many ways, online marketplaces are the perfect business model. Since they facilitate transactions between independent suppliers and customers rather than take possession of and responsibility for the products or services in question, they have inherently low cost structures and fat gross margins. They are highly defensible once established, owing to network effects. Yet online marketplaces remain extremely difficult to build, say Andrei Hagiu of Harvard Business School and venture capitalist Simon Rothman of Greylock Partners. Most entrepreneurs and investors attribute this to the challenge of quickly attracting a critical mass of buyers and suppliers. But it is wrong to assume that once a marketplace has overcome this hurdle, the sailing will be smooth. Several other important pitfalls can threaten marketplaces: growing too fast too early; failing to foster sufficient trust and safety; resorting to sticks, rather than carrots, to deter user disintermediation; and ignoring the risks of regulation. This article draws on company examples such as eBay, Lending Club, and Airbnb to offer practical advice for avoiding those hazards.

We study the economic tradeoffs that drive organizations to position themselves closer to or further away from a multi-sided platform (MSP) business model, relative to three traditional alternatives: vertically integrated firms, resellers or input suppliers. These tradeoffs lead to a comprehensive discussion of the defining features of MSPs. The formal model we develop focuses on the MSP versus vertical integration choice, which we interpret in the context of professional services. A key tradeoff emerges between the need to coordinate decisions that generate spillovers across professionals (best achieved by a vertical integrated firm) and the need to both motivate unobservable effort by professionals and ensure professionals adapt their decisions to their private information (best achieved by a MSP). We show how this baseline tradeoff is impacted by the nature of contracts available to the vertically integrated firm and the MSP, and by the possibility of professionals holding pessimistic expectations when deciding whether or not to join the vertically integrated firm or MSP.